The present invention relates to an optical data recording and reproduction apparatus and, more particularly, to an optical data recording and reproduction apparatus of the type using a semiconductor laser as a light source.
A predominant light source in the optical data recording and reproduction art is a semiconductor laser which is small in size and low in cost. The output power of a semiconductor laser necessary for recording data is about ten times greater than one necessary for reproduction. One approach heretofore proposed to ensure such output power for recording is increasing the numerical aperture, or NA, of a condenser lens, which is adapted to focus a beam issuing from a semiconductor laser. Specifically, the numerical aperture of a lens and the coupling efficiency .eta., or a ratio of light focused to parallel rays by the lens, are generally related as shown in FIG. 1. Optical pickups exclusively used for reproduction or playback purposes are usually implemented by condenser lenses whose numerical apertures are not greater than 0.2, i.e. 0.13-0.17. Although such numerical apertures are reflected by coupling efficiencies n as low as 20-30%, they offer sufficient allowances concerning the amount of light because of the exclusive use of the pickups for playback. While an increase in the light output may be accomplished by increasing the numerical aperture of a lens in order to provide an optical pickup with a recording capability as well as the playback capability, it cannot exceed a certain limit in consideration of the accuracy with which the lens should be mounted to a light source and the like. It is necessary, therefore, to enhance the output power of a semiconductor laser itself which functions as a light source.
An ordinary optical recording and reproduction apparatus using a semiconductor laser is shown in FIG. 2 and generally designated by the reference numeral 10. As generally accepted, the half angle .theta..sub.H of light issuing from a semiconductor laser 12 in a direction parallel to the junction direction of the laser lies in the range of 6-40 degrees. The current trend in the optical recording and playback art is to the use of a laser whose half angle .theta..sub.H is not greater than 10 degrees, so far as the high output power semiconductor laser 12 is concerned. If the half angle .theta..sub.H is small, a parallel beam passed through a condenser lens 14 will have a small diameter .PHI..sub.c and, therefore, a spot projected by an objective lens 16 onto a recording medium 18 will have its diameter increased relatively. The increase in the spot diameter at the recording medium 18 naturally results in inaccurate recording or playback.
In light of the above, it has been customary to interpose between the coupling lens 14 and the objective lens 16 beam trimming optics which includes, for example, a plano-concave cylindrical lens 20 and a plano-convex cylindrical lens 22, so that the beam diameter .PHI..sub.c is increased to in turn reduce the spot diameter. In FIG. 2, designated by the reference numeral 24 is a polarizing beam splitter, 26 a quarter-wave plate, and 28 a light receiving element for receiving light reflected by the recording medium 18.
The beam trimming, however, causes the objective 16 to limit the beam and, thereby, brings about a loss of quantity of light by 30-40% compared to a non-trimming condition. To compensate for the loss, a system featuring both the recording and playback capabilities has to be furnished with a semiconductor laser which produces much greater output power, at the cost of an expensive design of the whole apparatus.